db: make compatible with SQLAlchemy 0.8

Argument assert_unicode is deprecated in SQLAlchemy 0.7.
Also it is removed from SQLAlchemy 0.8.
Arguments convert_unicode, unicode_error, and _warn_on_bytestring
seem to cause problems with SQLAlchemy 0.8.

Closes-bug: 1235829

Change-Id: I713dd446756398805404400d05b1d0927601dd97
Merge "db: make compatible with SQLAlchemy 0.8"